This invention is in the field of drying cylinders; more particularly, the invention relates to an apparatus and method of operating drying cylinders for a papermaking machine.
Steam heated drying cylinders are used to dry formed webs of paper. Steam is fed into and condenses within the drying cylinders giving up its latent heat. The rotation of the drying cylinder forces the condensed steam against the inside of the cylindrical dryer wall. A means such as a syphon removes the condensate from the cylinder. In addition to condensate, steam also enters the syphon and leaves the drying cylinder. As discussed below, it is recognized in the art that it is desireable to control the amount of steam blow through. Until the present invention this has only been accomplished by the sizing of the apparatus involved with no continuous control.
A known method to control the amount of blow through steam is to place a carefully sized or designed orifice in the condensate discharge line. This permits the passage of a predetermined flow rate of condensate and the reduction of the level of steam blowing through to a desired minimum level. These orifices are desired for optimum dryer operation and cannot be continuously controlled. Such orifices are disclosed in U.S. Pat. Nos. 4,089,121 and 3,251,138.
Various systems, such as the one in U.S. Pat. No. 1,192,369, have been used to remove condensate from the drying cylinders. These systems comprise the creation of a pressure difference across the cylinder. In this way the desired condensate flow rate is obtained. However, no system continuously controls the amount of steam blown through with the condensate. In addition to merely creating a pressure difference, U.S. Pat. No. 2,811,787 attempts to achieve the optimum pressure difference to force out the condensate, which becomes a function of the velocity of the cylinder. Here no attempt is made to control the amount of blow through steam.
Presently, instantaneous measuring devices such as beta-ray scanners are used to continuously measure moisture in the paper and adjust the heat by controlling steam pressure accordingly. Other control schemes disclosed measure changes in steam pressure or condensate flow rate as an indication of the changing conditions of the paper or item being dried and signal the process to be adjusted accordingly. These latter control schemes are concerned with heat transferred to the paper and not with the control of the amount of blow through steam. Such systems are disclosed in U.S. Pat. Nos. 4,106,211; 2,208,784 and 3,251,138 which as noted above uses a carefully designed orifice to minimize instream blow through. Therefore, although many control schemes have been used none have been designed to control the amount of steam blow through.
Blow through steam can be recirculated as in U.S. Pat. Nos. 4,106,211 and 1,643,972 and reused at lower pressure in cascade drying cylinders as shown in U.S. Pat. No. 2,811,787.
The art indicates the importance of control in drying cylinders generally and for particularly minimizing the amount of blow through steam. The amount of blow through steam has been directly controled to minimum desired levels only by sizing equipment. It is desireable to continuously control the amount of blow through steam.